The present invention relates to television cameras, and more particularly to dynamic beam current control circuits for said cameras.
A television camera tube has a target which is illuminated by the scene that is viewed by the camera tube. The intensity of the light falling on the target causes an electrical discharge of it. An electron beam sweeps across the target and recharges it to a fixed value. The current that is required to recharge the target plate is proportional to the intensity of the light falling upon the target plate. If the camera tube is viewing a very intense object, the electron beam current may not be sufficient to totally recharge the target plate. This especially occurs when the camera and the viewed object are moving with respect to one another. It causes a visible effect known as "comet tail", which trails a brightly illuminated spot when said motion occurs.
The beam current which flows in the camera tube is a function of the voltage applied to its control electrode. This current traditionally has been a compromise between minimizing a comet tail effect, which requires a large current, and high resolution, which requires a low beam current. If the voltage on the control electrode can vary with the signal current created by the optical image, then the beam current can temporarily and automatically be increased to accomodate local areas of excess light. This technique has been named "dynamic beam current control". Such a beam current control scheme can, and has in the past, been accomplished by connecting the pre-amplified pick-up tube output signal back to the control electrode of the camera tube via ordinary electronic amplifiers. This method however exposes several undesirable side effects which can be annoying when viewing the displayed picture. First propagation delay of the signal through the electronic amplifier leads to a ghost image in the picture. This is due to crosstalk between the control and signal target electrodes within the pick-up tube. Second the control loop tends to be unstable because the control electrode voltage versus cathode beam current characteristic of the pick-up tube is non-linear. Typically it follows a square law.